The present invention relates to an electrooptical device for controlling and monitoring the wavelength and emission spectrum of a tunable laser. More specifically, the invention relates to a system designed to be fitted in the cavity of a tunable laser and, controlled by a very small electrical control signal, to tune a laser emission line wavelength over a wide frequency range, in a wavelength domain going from ultra-violet to infrared. The system is designed to tune the wavelength according to an arbitrary wavelength scanning law, at a speed that can be high.
Usually, wavelength tuning devices in tunable lasers such as continuous dye lasers are birefringent filters or diffractive gratings that alter the emission wavelength by rotating the filter in the cavity. The advantage of these systems is their simple design and the narrowness of the emission line they produce. One disadvantage is their low tuning speed, which is limited by the mechanical inertia of the system itself. Another disadvantage is that it is impossible to change the wavelength discontinuously, i.e. to switch from one wavelength to another without going through the intermediate wavelengths.
A second type of wavelength tuning system is the acousto-optical type in which an acoustical wave is used to vary the emission wavelength. The disadvantage here comes from the fact that the system introduces large losses, and can thus be used only with pulsed lasers. Another disadvantage relates to the narrow tuning range, limited to a few angstroms.
A third type of wavelength tuning system comprises electrooptical systems consisting of electrooptical crystals. Different crystal cuts adapted to tunable laser wavelength scanning have been proposed, for example the 45.degree.Z cut or 0.degree.Z cut with transverse electric field. The wavelength is then controlled by an electric signal applied to the electrodes. The disadvantage of this configuration is that the required control voltage is several kilovolts, which limits the use of these systems to tuning spectra of a few angstroms, for technological reasons. Other configurations, such as crystal cuts X45.degree. and 0.degree.Z with longitudinal electrical field, have been proposed. The wavelength is then tuned electrically by inclining the wavelength tuning system inside the laser cavity. In those configurations proposed up till now, the control voltages needed are still relatively high, of the order of a few hundred volts for Rhodamine 6G. The power supplies needed for controlling all of these systems become more sophisticated and costly as the tuning range is extended and the tuning rate is increased. Moreover, the limitation may be introduced, for example in systems including a 0.degree.Z crystal cut with longitudinal electric field, by piezoelectric resonances that make it impossible to change the wavelength at audiofrequency rates.